Add the potatoes and cauliflower to a pot and add enough water to cover. Bring the water to boiling, and then let simmer until the potatoes and cauliflower are tender.
Preheat the oven to 400 degrees Fahrenheit.
Meanwhile, heat the butter or oil in a large oven-safe pan. Saute the carrots and onions for about 5 minutes, and then add the beef and saute until cooked. Add peas and cook for an additional 2-3 minutes.
Season the vegetables and meat with salt, pepper, thyme, and juniper. Add the beer and raise the temperature to medium-high. Add the flour and stir for 2-3 minutes or until the flour cooks and the beer thickens into a sauce. Remove from heat.
Mash the potatoes with half-and-half and salt and pepper if desired.
Spread the vegetables and meat evenly across the pan and top with the mashed potatoes. Sprinkle with the cheese and then bake for 15 minutes.
Let rest 5-10 minutes before serving.
Notes
If you buy your corned beef in large portions from the butcher, it should come with whole juniper berries. If not, you can find dried juniper berries in the spice aisle or simply omit them from the recipe.